1. Field of the Invention
The present invention relates to, for example, a small-sized vehicle such as a saddle type three-wheel or four-wheel vehicle used for farming, an ATV (All Terrain Vehicle) for sportive traveling off-road, a golf cart, and other small-sized vehicles. More specifically, the present invention relates to a small-sized vehicle having an electric motor-driven power steering apparatus.
2. Description of the Related Art
Generally, a small-sized vehicle used for traveling off-road has the following structure. Front and rear wheels each having a balloon tire with low pressure and wide diameter are disposed at the front and rear of a vehicle body frame, and an engine unit is mounted between the front and rear wheels. Further, a steering shaft is arranged between the left and right front wheels, and a wheel-type steering handle or a bar-type steering handle is attached to the upper end of the steering shaft. A small-sized vehicle of this type can travel off-road at will by turning the steering handle from side to side.
In recent years, some users expect further operational improvements when maneuvering the small-sized vehicle so as to reduce a load on operating the steering handle.
Accordingly, an electric motor-driven power steering apparatus which is connected to a steering shaft to thereby supply the steering shaft with an assisting steering force from the electric motor has been suggested.
In the power steering apparatus, under steering operations, the electric motor always operates and generates heat. When traveling at high speeds, a steering angle or a steering speed is rarely increased, such that the amount of heat generated by the electric motor is relatively small. In addition, when traveling at high speeds, the electric motor is cooled by moving air, and thus, requires less cooling. In contrast, when traveling at low speeds, the steering speed and angle are often increased. In that case, the amount of heat generated is increased, and the amount of cooling provided by the moving air is decreased. Accordingly, the electric motor requires increased cooling when traveling at low speeds.